For the uninitiated, Helldivers 2 is a squad-based shooter, where you and up to three team-mates drop onto a planet to achieve your mission objectives before being airlifted to safety. These objectives range from 'take this thing here', 'blow this thing up' or 'defend this bit of ground while a thing happens', through to 'kill as many enemies as you can while staying as alive as you can'. It's safe to say that it's not a story-heavy game.
However, there is some framing. You are a Helldiver, one the elite soldiers of Super Earth, sent to battle its enemies while championing liberty, spreading Managed Democracy and staying true to the cause. Of course, it's all camouflage over the fact that you fight for a totalitarian dictatorship, with the satire very reminiscent of Starship Troopers.